Add parametrized test for verifying parsing and writing.
Make a single data driven unit test as a template for additional tests.

Every test file is now verified for identity after parsing and writing,
and then for additional test specific properties using a custom
VerificationMethod for each file. The VerificationMethod is specified in
the test row data.
Add test data for clock entries and extend test.

The test of course fails for now :-)
Add Clock class. Clock operates on clock entries for elements.
Implement CLOCK: line detection and parsing.
Implement Clock::duration(). Extend test.